EDELWEISS-II : Status and future Véronique SANGLARD CNRS/IN2P3/IPNLyon sanglard@ipnl.in2p3.fr http://edelweiss.in2p3.fr

Outline Context EDELWEISS-I EDELWEISS-II Conclusion Run with heat trigger Final results Background studies EDELWEISS-II Improvements Present and future runs Conclusion Dark Matter 2006 – Marina del Rey, California – 02/24/06 « EDELWEISS-II : Status and future » Véronique SANGLARD

The EDELWEISS Collaboration CEA-Saclay DAPNIA/DRECAM CNRS/CRTBT Grenoble CNRS/IN2P3/CSNSM Orsay DUBNA (Russia) FZK/Univ. Karlsruhe (Deutschland) CNRS/INSU/IAP Paris CNRS/IN2P3/IPN Lyon CNRS – CEA/Laboratoire Souterrain de Modane 1700 m depth under the Fréjus tunnel (4800 we) 4 µ/m²/d (106 less than at the surface) *Expérience pour DEtecter Les WIMPs En SIte Souterrain (Underground experiment to detect WIMP) Dark Matter 2006 – Marina del Rey, California – 02/24/06 « EDELWEISS-II : Status and future » Véronique SANGLARD

Heat and Ionization Ge detectors Center electrode Guard ring 7 cm Reference electrode Thermometer (Ge NTD) Fiducial volume(≈ 57%) Ge crystal Guard Electrodes Center electrode Ionization guard Simultaneous measurement of Heat @ 17 mK with Ge/NTD sensor Ionization @ few V/cm with Al electrodes Different charge/heat ratio for nuclear and electron recoils (WIMP and neutron have lower charge than γs, βs ) Discrimination event-by-event of electron recoils (main background) EI/ER = 0.3 for nuclear recoils EI/ER = 1 for electronic recoils m=320g Ionization center Ionization threshold Neutrons 73Ge(n,n',γ) Gammas Dark Matter 2006 – Marina del Rey, California – 02/24/06 « EDELWEISS-II : Status and future » Véronique SANGLARD

2003 data taking with heat trigger Several runs between 2000 and 2003 Last run = data taking with trigger on heat signal Improved efficiency at low energy (50 % at 11 keV) Stable behavior over 4 months of the detectors Fiducial exposure: 22 kg.d 18 nuclear recoil candidates > 15 keV Possible backgrounds Residual neutron flux 1 n-n coincidence observed 2 single expected by MC Surface electron recoils Miscollected charge events at low energy Leak of events down to the nuclear recoil band not visible in coincidence events Further, studies concerning the possible origins for these backgrounds Dark Matter 2006 – Marina del Rey, California – 02/24/06 « EDELWEISS-II : Status and future » Véronique SANGLARD

Final results of EDELWEISS-I (1) Final results: 62 kg.d (fid. exp.) Overall efficiency @ 15 keV = 50% 40 nuclear recoil candidates > 15 keV Only 3 with 30 <ER< 100 keV  strong constraint on an eventual WIMP rate Unknown background Used method developed by S. Yellin to derive exclusion limits (as CDMS) *(PRD 66,032005 (2002)) No background subtraction V.Sanglard et al., PRD 71,122002 (2005) Experiment stopped in March 2004 Dark Matter 2006 – Marina del Rey, California – 02/24/06 « EDELWEISS-II : Status and future » Véronique SANGLARD

Exclusion limit – Spin Independent New exclusion consistent with our previous limits Best sensitivity : 1.5x10-6 pb @ 80 GeV/c² EDELWEISS starts to explore some optimistic SUSY models EDELWEISS-II EURECA Need a gain in sensitivity of a factor 100 – 10000 (EDW-II, EURECA*) to explore more interesting SUSY models This gain depends on improvements On background discrimination On mass (* See Hans Krauss’s talk) Dark Matter 2006 – Marina del Rey, California – 02/24/06 « EDELWEISS-II : Status and future » Véronique SANGLARD

Exclusion limit – Spin Dependent In some models, the spin dependent interactions dominate 7.8 % of 73Ge (J=9/2) in natural Ge Spin on neutron or proton results present in (ap, an) diagram ap, an = effective coupling between WIMP and proton, neutron For each WIMP mass, the constraint is Optimisitic SUSY models are a factor 100 below Direct detection experiments not yet competitive with indirect detection (SK) Dark Matter 2006 – Marina del Rey, California – 02/24/06 « EDELWEISS-II : Status and future » Véronique SANGLARD

g-ray background in EDELWEISS-I Before nuclear recoil selection, rate in detectors is ~1.5 event/kg/day/keV at low energy At high energy, spectrum shape and rate consistent with simulations of the measured U/Th contamination in the bulk of the Cu shielding Room for improvement in EDELWEISS-II, where this Cu is not used Dark Matter 2006 – Marina del Rey, California – 02/24/06 « EDELWEISS-II : Status and future » Véronique SANGLARD

Neutron background in EDELWEISS-I Recent estimate of the neutron flux in the lab with E >1 MeV : 1.1  0.1(stat) 10-6 n/cm²/s Good understanding of neutron propagation in the setup (agreement between simulated and experimental spectrum for a neutron calibration) Determination of single rate : ~2 nuclear recoil expected in 62 kg.d (ambient radioactivity + U contamination of copper and lead) 1 n-n coincidence observed in 62 kg.d Not a strong constraint on the single neutron rate in the data Expected ratio double/single ~1/10 1 n-n  1 – 40 singles with ER>15 keV @ 90% C.L. Indistinguishable from the miscollected events in the nuclear recoil band Dark Matter 2006 – Marina del Rey, California – 02/24/06 « EDELWEISS-II : Status and future » Véronique SANGLARD

Surface backgrounds in EDELWEISS-I Peak at E=5.3 MeV a’s from 210Po? Q=0.3  a decays near surfaces Rate ~ 400 /m²/d As expected, non-fiducial part more exposed to a flux Very likely due to 210Pb on Cu or Ge surfaces No 206Pb recoil peak at 100 keV observed as heat-only events : 210Pb implanted in Cu, not Ge. Rate of 0.3<Q<1.0 events at low energy consistent with surface b’s expected in 210Pb hypothesis (but does not exclude possible contribution from 14C) By removing Cu covers between detectors Events should disappear Better identification by coincidences E = 5.3 MeV, Q = 0.3 1 Dark Matter 2006 – Marina del Rey, California – 02/24/06 « EDELWEISS-II : Status and future » Véronique SANGLARD

EDELWEISS-II in few words Installation in the LSM started summer 2004 1rst funded stage : 28 detectors 21*320g optimized Ge/NTD detectors and holders 7*400g Ge/NbSi detectors with active surface events rejections First cryogenic test with bolometers in january 2006 Commissioning run with 8 bolometers New electronic and acquisition systems: square modulation, continuous digitization close to the readout, optical fibers and numerical trigger Goal *100 in sensitivity : w-n 10-8 pb 0.002 evt/kg/day (ER>10keV) Dark Matter 2006 – Marina del Rey, California – 02/24/06 « EDELWEISS-II : Status and future » Véronique SANGLARD

EDELWEISS-II improvements – Cryostat Reversed geometry Nitrogen free : 3 Pulse tube (50K and 80K screens) and 1 He cold vapor reliquefier (consumption  0) Large volume 50l  Self shielding Up to  120 detectors  More statistics Compact and hexagonal arrangement  More coincidence (n bkg) Dark Matter 2006 – Marina del Rey, California – 02/24/06 « EDELWEISS-II : Status and future » Véronique SANGLARD

EDELWEISS-II improvements – Backgrounds Radiopurity Dedicated HPGe detectors for Systematic checks off all materials Clean Room (class 100 around the cryostat, class 10000 for the full shielding Deradonized air (from NEMO3) (0.1 Bq/kg) 20 cm Pb shielding Neutron Shielding EDW-I : 30cm paraffin EDW-II : 50 cm PE and better coverage µ veto (99% coverage) Dark Matter 2006 – Marina del Rey, California – 02/24/06 « EDELWEISS-II : Status and future » Véronique SANGLARD

EDELWEISS-II improvements – Ge/NTD detectors Developed by CEA Saclay and Camberra-Eurisys Amorphous Ge and Si sublayer (better charge collection for surface events) Optimized NTD size and homogeneous working T (16-18 mK) : sub keV resolution New holder and connectors (Teflon and copper only) Dark Matter 2006 – Marina del Rey, California – 02/24/06 « EDELWEISS-II : Status and future » Véronique SANGLARD

EDELWEISS-II improvements – surface events Important limitation to the sensitivity : due to diffusion, trapping and recombination, surface events are miscollected and can mimic nuclear recoils 2 approaches : Passive rejection : improve the charge collection for surface event Physics of the charge collection, trapping, surface charge, regeneration Physics of the Ge and Si amorphous sublayer Detectors with thick electrodes Active rejection : identification of the surface events Pulse shape analysis of the charge signals (but high bandwidth  low noise)  localization of the event Interdigitzed electrodes Detectors sensitive to athermal phonons  Ge/NbSi detectors Dark Matter 2006 – Marina del Rey, California – 02/24/06 « EDELWEISS-II : Status and future » Véronique SANGLARD

Identification of surface events with Ge/NbSi detector Identification of near surface events using athermal phonon measurement with NbSi thin film thermometers Heat and ionization Ge detectors Each signal = thermal + athermal component For surface events, athermal higher in NbSia Thermal signals proportionnal to the deposited energy Tests with 200g prototype in EDELWEISS-I Improvement of a factor 20 of the rejection Fiducial volume reduction of 10 % Before rejection After rejection (1mm cut) Dark Matter 2006 – Marina del Rey, California – 02/24/06 « EDELWEISS-II : Status and future » Véronique SANGLARD

EDELWEISS-II present status Commissioning run with 8 bolometers : 2*320g Ge/NTD with EDW-I holder 2*320g Ge/NTD with EDW-II holder and teflon clamp 2*320g Ge/NTD with EDW-II holder and Cu springs 1*IAS 50g heat and light detector (Al2O3) 1*200g Ge/NbSi Goals : Validation of the microphonics (pulse tube decoupling system), new holders and new comb connectors for Ge/ntd, new electronics scheme, new acquisition system… Dark Matter 2006 – Marina del Rey, California – 02/24/06 « EDELWEISS-II : Status and future » Véronique SANGLARD

EDELWEISS-II future Feb-march 2006 : continue present run = commissioning April 2006 : EDELWEISS-II with 28 detectors (21 Ge/NTD and 7 Ge/NbSi) + IAS detector (heat and light) 2006 : Approval of the 2nd stage : 120 detectors In term of sensibility : EDELWEISS-II : 1 evt/kg/year EURECA : 10 evt/kg/year Dark Matter 2006 – Marina del Rey, California – 02/24/06 « EDELWEISS-II : Status and future » Véronique SANGLARD